The functional role of RNase H activity of reverse transcriptase during viral replication has been investigated by labeling the 5' end of high molecular weight viral RNA by two procedures: (1) exchange reaction with (32P) ATP and polynucleotide kinase; (2) a series of enzymatic reactions which involve decapping of the 5' "cap" structure with decapitase, alkaline phosphatase and kinase. Preliminary data suggested that the 5' end of template RNA was not cleaved during reverse transcription. Thus RNase H activity must function at the other stages of proviral synthesis. Interaction and conformation of viral RNA within the intact virion was examined by isolating ribonucleoprotein complexes (RNP). These complexes are important in biological processes in which the viral genome participates. The protein content of these complexes as well as their cDNA products are currently being investigated and compared.